1. Field of the Invention
The present invention relates to a television tuner and in particular to a television tuner equipped with a coupling varactor diode.
2. Description of the Related Art
FIG. 8 shows a circuit diagram of a related art television tuner. A television signal in the VHF band to UHF band is input to a VHF input tuning circuit 22 and a UHF input tuning circuit 23 via a trap circuit 21 for removing unnecessary signals such as an FM broadcast signal.
The VHF input tuning circuit 22 has a tuning varactor diode 22a whose anode is grounded, four coils sequentially connected serially with the cathode thereof via high frequencies and connected in parallel with the tuning varactor diode 22a via high frequencies, that is, a high-band coil 22b, a low-band coil 22c, a low-band coil 22d and a high-band coil 22e, and a switch diode 22f connected in parallel across two low-band coils 22c and 22d via high frequencies. Inductance of the high-band coils 22b, 22e is smaller than that of the low-band coils 22c and 22d. The high-band coil 22e on the low potential side is grounded via high frequencies by a DC cut capacitor 22g. To the connection point of the two low-band coils 22c and 22d is input a television signal. The connection point of the cathode of the tuning varactor diode 22a and the high-band coil 22b serves as an output end of the VHF input tuning circuit 22.
The connection point of the high-band coil 22e and the DC cut capacitor 22g is grounded by a resistor 22h and connected to a power terminal B by a resistor 22i. A power voltage of 5 volts is applied to the power terminal B. A bias voltage divided by the resistors 22h and 22i is applied to the anode of the switch diode 22f via the high-band coil 22e and the low-band coils 22d, 22c. The cathode of the switch diode 22f is connected to a band switching terminal Sw. A high-level (5-volt) or low-level (0-volt) switching voltage is applied to the band switching terminal. The cathode of the tuning varactor diode 22a is connected to a tuning voltage terminal Tu via the high-band coil 22b. A tuning voltage is applied to the tuning voltage terminal Tu.
In a stage subsequent to the VHF input tuning circuit 22 is provided a VHF high-frequency amplifier 24 comprising an FET (Field Effect Transistor). The VHF input tuning circuit 22 and the VHF high-frequency amplifier 24 are coupled to each other via a coupling varactor diode 25. The coupling varactor diode 25 has the same characteristics as those of the tuning varactor diode 22a. The cathode of the coupling varactor diode 25 is connected to the cathode of the tuning varactor diode 22a. The anode of the coupling varactor diode 25 is grounded by a resistor 26 and coupled to the input end of the VHF high-frequency amplifier 24 via a DC cut capacitor 27.
A capacitor 28 is connected between the connection point of the two low-band coils 22c, 22d and the anode of the coupling varactor diode 25. A capacitor 29 is connected between the connection point of the high-band coil 22b and the low-band coil 22c and the anode of the coupling varactor diode 25.
In a stage subsequent to the VHF high-frequency amplifier 24 is provided a VHF inter-stage tuning circuit 40, subsequently to which is provided a VHF mixer (not shown). The VHF inter-stage tuning circuit 40 and the subsequent configuration will not be described.
A UHF input tuning circuit 23 has a coil 23a one end of which is grounded, and two tuning varactor diodes 23b, 23c of the same characteristics whose cathodes are connected to each other. The anode of one tuning varactor diode 23b is connected to the other end of the coil 23a. The anode of the other tuning varactor diode 23c is grounded by a DC cut capacitor 23d and a resistor 23e. The connection point of the coil 23a and the anode of the tuning varactor diode 23b is connected to a trap circuit 21 via a DC cut capacitor 41. The cathodes of the two tuning varactor diodes 23b, 23c are connected to the tuning voltage terminal Tu and to a UHF high-frequency amplifier 43 by a DC cut capacitor 42. An FET is used also in the UHF high-frequency amplifier 43.
In a stage subsequent to the UHF high-frequency amplifier 43 is provided a UHF inter-stage tuning circuit 44, subsequently to which is provided a UHF mixer (not shown). The UHF inter-stage tuning circuit 44 and the subsequent configuration will not be described.
In the aforementioned configuration, in order to receive a television signal in the VHF band, the VHF high-frequency amplifier 24 is placed in the active state while the UHF high-frequency amplifier 43 is placed in the inactive state. In order to receive a television signal in the UHF band, the UHF high-frequency amplifier 43 is placed in the active state while the VHF high-frequency amplifier 24 is placed in the inactive state.
In the case of receiving a high-band television signal in the VHF band, a voltage at the band switching terminal Sw is driven low. This turns on the switch diode 22f and the VHF input tuning circuit 22 serves as an equivalent circuit shown in FIG. 9. A coil 22j in FIG. 9 shows two low-band coils 22c, 22d in FIG. 8 connected in parallel.
A tuning frequency is mainly determined by two high-band coils 22b, 22e and a tuning varactor diode 22a. A tuning voltage within the range of about 2.5 to 25 volts is applied to the cathode of the tuning varactor diode 22a, and accordingly a tuning frequency varies in the high band. The capacitance value of the tuning varactor diode 22a obtained when the tuning voltage is 2.5 volts is about 14.5 pF (picofarad) which is tuned to the lowest frequency of the high band. The capacitance value is 2 to 3 pF when the tuning voltage reaches 25 volts.
As understood from FIG. 9, a series/parallel-resonance circuit comprises a capacitor 28, a coil 22j, a high-band coil 22b, and a coupling varactor diode 25. The series/parallel-resonance circuit serves as a trap circuit for attenuating image frequencies that accompany incoming high-band signals.
Similarly, another series/parallel-resonance circuit comprises a capacitor 29, a high-band coil 22b, and a coupling varactor diode 25. The series/parallel-resonance circuit serves as a trap circuit for attenuating the higher frequencies in the UHF band.
Both circuits are used for countermeasure against broadcasting interference.
In the case of receiving a low-band television signal in the VHF band, a voltage at the band switching terminal Sw is driven high. This turns off the switch diode 22f and the VHF input tuning circuit 22 serves as an equivalent circuit shown in FIG. 10. A tuning frequency is mainly determined by two high-band coils 22b, 22e, two low-band coils 22c, 22d, and a tuning varactor diode.
In this case also, in the VHF input tuning circuit 22, a tuning frequency varies in the low band in accordance with a tuning voltage within the range of about 2.5 to 25 volts applied to the cathode of the tuning varactor diode 22a. The tuning frequency is tuned to the lowest frequency of the low band when the tuning voltage is 2.5 volts.
Further, a series/parallel-resonance circuit comprising a capacitor 28, a low-band coil 22c, a high-band coil 22b, and a coupling varactor diode 25 serves as a trap circuit for attenuating image frequencies that take place when low-band signals are received by using parallel resonance frequencies.
FIG. 11 shows a circuit diagram of another related art television tuner. An input tuning circuit 31 to which a television signal in the VHF band is input comprises a single tuning circuit, in which a switch diode 31a and a varactor diode 31b are provided. The switch diode 31a is switched on/off and accordingly the input tuning circuit 31 is switched so as to be tuned to the high band or low band of the VHF band. A tuning voltage is applied to a tuning varactor diode 31b by the tuning voltage terminal Tu. The anode of the tuning varactor diode 31b is grounded. By varying the tuning voltage applied to the cathode, the tuning frequency varies in each band.
In a stage subsequent to the input tuning circuit 31 is provided a high-frequency amplifier 32. An FET (not shown) is used as its amplifying element.
In a stage subsequent to the high-frequency amplifier 32 is provided a double tuning circuit 33. A simple configuration of the double tuning circuit 33 is shown. On the preliminary side of the double tuning circuit 33 are provided a high-band coil 33a and a low-band coil 33b serially connected to each other, a tuning varactor diode 33c connected in parallel with these coils, and a switch diode 33d connected in parallel with the low-band coil 33b. 
Similarly, on the secondary side are provided a high-band coil 33e and a low-band coil 33f serially connected to each other, a tuning varactor diode 33g connected in parallel with these coils, and a switch diode 33h connected in parallel with the low-band coil 33f. The anodes of the two tuning varactor diodes 33c, 33g are grounded. A tuning voltage is applied on the cathode thereof by the tuning voltage terminal Tu. Two switch diodes 33d, 33h are switched on/off and accordingly the double tuning circuit 33 is switched so as to be tuned to the high band or low band. By varying the tuning voltage applied to the cathodes of the tuning varactor diodes 33c, 33g, the tuning frequency in each band is varied.
In a stage subsequent to the double tuning circuit 33 is provided a mixer for frequency conversion 34. The mixer 34 is coupled to the double tuning circuit 33 via a coupling varactor diode 35. The cathode of the coupling varactor diode 35 is connected to the cathode of the tuning varactor diode 33g on the secondary side. The anode of the coupling varactor diode 35 is connected to the ground as well as to the mixer 34 via a DC cut capacitor 37. A local oscillating signal is provided to the mixer 34 from a local oscillator 38.
In the aforementioned configuration, the tuning varactor diode 31b in the input tuning circuit 31, two tuning varactor diodes 33c, 33g in the double tuning circuit 33, and the coupling varactor diode 35 have the same characteristics. The voltage/capacitance characteristics are such that the inclination of a variation in the capacitance value is steep in a range where the tuning voltage is lower while the inclination of a variation in the capacitance value is gentle in a range where the tuning voltage is higher, as shown in FIG. 12.
The variable range of the tuning voltage applied to the cathode of each of the tuning varactor diodes 31b, 33c, 33g and the coupling varactor diode 35 is determined by the range of a tuning frequency required for each tuning circuit 31, 33, and is about 2.5 volts to 25 volts for the high band and about 2.5 volts to 21 volts for the low band. That is, a tuning voltage of 2.5 volts is tuned to the lowest frequency in each band. The capacitance value of each of the tuning varactor diodes 31b, 33c, 33g and the coupling varactor diode 35 is about 14.5 pF (picofarad) as shown in FIG. 12.
A television signal roughly selected by the input tuning circuit 31 is amplified by the high-frequency amplifier 32. The resulting television signal is further steeply selected by the double tuning circuit 33 then input to the mixer 34. The television signal is mixed with a local oscillating signal in the mixer 34 and an intermediate frequency signal is output.
In the configuration shown in FIG. 8, the inductance of the high-band coil 22b is negligible in receiving a low-band signal in the VHF band so that the capacitor 29 is connected in parallel with the coupling varactor diode 25. Thus, the coupling volume of the VHF input tuning circuit 22 and the VHF high-frequency amplifier 24 increases and in particular a gain in a high frequency channel in the low band becomes higher thus increasing the gain deviation.
The capacitor 29 is connected in parallel with the coupling varactor diode 25. This causes a problem that tracking of the parallel resonance frequency of a series/parallel-resonance circuit comprising a capacitor 28, a low-band coil 22c, a high-band coil 22b, and a coupling varactor diode 25 and the tuning frequency of the VHF tuning circuit 22 is misaligned.
The invention thus aims at reducing a gain deviation in receiving low-band signals as well as aligning the tracking of the resonance frequency of a trap circuit for attenuating image frequencies and the tuning frequency of a VHF input tuning circuit.
Generally speaking, the gain of a television tuner decreases as the frequency in the receiving channel decreases, starting from the state where Q of each tuning circuit and impedance between stages are matched. In the low band, for example, the gain is about 38 dB at the frequency in the lowest-frequency channel where the tuning voltage is 2.5 volts and about 42 dB in case the tuning voltage is 21 volts, the resulting gain deviation in the same band being as large as 4 dB, as shown in FIG. 13.
The NF (noise figure) is 6.3 dB in the lowest-frequency channel and 4.4 dB in case the tuning voltage is 21 volts, showing a worse NF characteristic in the lower side.
In order to correct such a gain deviation in the same band and poor NF, a varactor diode with larger capacitance value at the minimum tuning voltage (2.5 volts) and a larger ratio to the capacitance value obtained when the tuning voltage has dropped may be used to provide tight coupling of the double tuning circuit 33 and the mixer 34 in lower frequencies. Such a varactor, however, is costly.
The invention thus aims at leveling a gain deviation in the same band by using a varactor diode having the same characteristics as those of the varactor used in a double tuning circuit for coupling a double tuning circuit to a mixer, without using a costly and special varactor diode.